The present invention relates to an ion implantation technique for use in a semiconductor device manufacturing process or the like and, more particularly, is concerned with a low-energy ion implantation technique for forming a shallow junction in a semiconductor substrate.
As semiconductor devices have been becoming more highly integrated and finer in recent years, there is a demand for forming shallow source/drain regions and thin gate insulating films, i.e., forming shallow junctions. For responding to such a demand, a low-energy ion implantation technique which implants ions at a low implantation energy has conventionally been employed, whereby ion implantation under a very low energy of 0.2 keV has recently become possible.
If the implantation energy is lowered, on the other hand, then ions are implanted deeper due to a channeling phenomenon. Recently, for preventing such a phenomenon, a preamorphization technique has also come into practice, which comprises the steps of implanting ions which form no electric carriers, such as germanium, argon, silicon or xenon, beforehand so as to cause a surface of a semiconductor substrate to have an amorphous state; and then continuously implanting aimed ions.
In the ion implantation with a very low energy, however, the projected range of ions is so shallow that the ions may be trapped in a naturally oxidized film formed on the substrate surface without reaching the inside of the semiconductor substrate. The atoms trapped by the naturally oxidized film are eliminated together with the naturally oxidized film in a cleaning process subsequent to the ion implantation process. Due to this mechanism, the implanted ion dose (effective dose) remaining in the semiconductor substrate after cleaning becomes smaller than the dose actually implanted in the ion implantation process or the measured dose (calculated dose). Also, the effective dose fluctuates since the thickness of naturally oxidized film fluctuates under the influence of the process immediately prior to the ion implantation.
Therefore, the surface of semiconductor substrate has conventionally been cleaned immediately before the semiconductor substrate is loaded into an ion implantation apparatus.
In practice, however, the effective dose may fluctuate for some unknown reason even in the above-mentioned process, i.e., the process in which the preamorphization ion implantation and the very-low-energy ion implantation of the aimed ions are continuously carried out after cleaning the semiconductor substrate (eliminating the oxidized film).
Hence, it is an object of the present invention to provide an ion implantation method which can accurately control the effective dose even upon ion implantation at a very low energy.
For achieving the above-mentioned object, the inventors have diligently studied the above-mentioned conventional ion implantation process and, as a result, has found that, in the preamorphization ion implantation process, oxygen existing within the ion implantation apparatus is implanted into the semiconductor substrate by ion beams, so as to form an oxidized film. The oxygen existing within the ion implantation apparatus includes that adsorbed onto the surface of semiconductor substrate during the transportation after cleaning, and that desorbed or floated from the inner wall face of the ion implantation apparatus upon heating by ion beam sputtering, beam power, and the like after having been adsorbed thereon. The inventors have found that the oxygen film thus formed in the preamorphization ion implantation process also influences the effective dose, since it traps ions in the subsequent process and is eliminated by the cleaning process after the ion implantation as with the naturally oxidized film.
The present invention is achieved according to such findings, and comprises the steps of carrying out preamorphization ion implantation for causing a surface of a semiconductor substrate to attain an amorphous state; then cleaning the surface of semiconductor substrate subjected to preamorphization ion implantation so as to eliminate an oxidized film; and thereafter carrying out ion implantation under a low implantation energy so as to form a shallow junction in the semiconductor substrate. As a consequence, the influence of the oxidized film formed by preamorphization ion implantation can be suppressed.
The semiconductor substrate is effectively cleaned with hydrofluoric acid.
In general, the semiconductor substrate is exposed to atmosphere after cleaning the semiconductor substrate until the low-energy ion implantation is started if common equipment is used. Therefore, it will be effective if the period of time after the cleaning until the starting of low-energy ion implantation is made constant. This is because of the fact that, in such a case, the amount of oxygen attaching to the semiconductor substrate becomes constant, so that the thickness of the oxidized film formed upon low-energy ion implantation, and the amount of ion trapped therein, in turn, become substantially constant, whereby the effective dose can be controlled more reliably.
Preferably, the ion species employed in preamorphization ion implantation is germanium, silicon, argon or xenon. Preferably, the ion species employed in the low-energy ion implantation is boron, and the implantation energy is about 2 keV or lower.
These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the attached drawings.